1. Field of the Invention
This invention relates to an ultrasonic actuator such as an ultrasonic motor (a vibration type motor) or the like, and more particularly to a drive control device for controlling an ultrasonic actuator so that the ultrasonic actuator can be driven in an optimum condition.
2. Related Background Art
Generally an ultrasonic actuator typified by an ultrasonic motor (a vibration type motor is such that an electro-mechanical energy conversion element such as a piezo-electric element or an electrostrictive element is adhesively secured to a side of an elastic member formed into a circular ring or an elliptical shape and an individual voltage is applied to the two drive phases of said element to thereby form a travelling vibration wave in said elastic member so that said elastic member and a member which is in pressure contact with said elastic member may be moved relative to each other by the travelling wave).
The elastic member is fixed, for example, to the fixed barrel side of a lens barrel and the member which is in pressure contact with the elastic member is defined as the focus ring side of a focusing lens, whereby the ultrasonic actuator can be used as a drive source for auto focusing (AF), and the elastic member is fixed to a printer body and the member which is in pressure contact with the elastic member is provided by a sheet such as paper, whereby the ultrasonic actuator can be used as a sheet feeding mechanism for feeding the sheet to the printing station of the printer. Further, the elastic member is mounted on the printer body for reciprocal movement along the widthwise direction of the sheet and a printing head is mounted on the elastic member, and the elastic member is brought into pressure contact with a rail-like stator fixed to the printer body along the direction of movement of the elastic member, whereby it can be used as a carriage mechanism for effecting the scanning of the printing head.
Also, a construction similar to the carriage mechanism can be used as a driving mechanism for a precision stage.
These mechanisms utilizing an ultrasonic actuator can directly move the elastic member and the member which is in pressure contact with this elastic member without the intermediary of a transmission mechanism such as gears and therefore is suitable for a mechanism for feeding sheets highly accurately at a predetermined pitch, such as, for example, a sheet feeding mechanism in a printer.
In the vibration wave actuator, a member moved relative to the elastic member, for example, a sheet, is in pressure contact with the elastic member and therefore, the holding torque is great during the non-application of an electric field to the drive phase of the piezo-electric element and thus, design is made such that when the sheet is intermittently fed, an electric field is applied to the drive phase during the start of the sheet feeding and that during the stoppage of the sheet feeding, a drive power source is opened to stop the application of the electric field to the drive phase.
Now, there is an advantage that, for example, the sheet can be brought to a standstill simply by opening the power source during the stoppage of the sheet fading. However, while the temperature of the ultrasonic actuator falls during the stoppage and the temperature rises for several to several tens of seconds during the re-starting and, therefore, the vibration performance (response characteristic, etc.) of the vibration member comprising the piezo-electric element, the elastic member and the adhesive agent or the like therebetween, and the torque transmission performance of a friction material vary greatly. In view of this, it has been proposed to effect high-degree control, taking the response characteristic of each temperature into account and maintaining the performance (T-N characteristic, etc.) of the actuator constant. However, in the case of such a control system, if speed control or position control is immediately effected during the start-up, the influence of the temperature of the atmosphere and the temperature distribution in the motor will not be stable, and this has led to the problem of low accuracy.
It has also been found from experiments that too low temperature of the vibration member tends to aggravate the performance (T-N characteristic, etc.) of the actuator.